In recent years, “wide area Ethernet® service” that provides a network service using versatile Ethernet® equipment has come to be used as the backbone of an intra-enterprise network by an increasing number of users since it is inexpensive and there are no restrictions on the protocols that can be used.
However, because of its low cost and versatility, Ethernet® equipment is prone to failure, and is therefore not suited to accommodate lines that carry data such as bank account data for which high reliability is demanded. Accordingly, there is a need for a network configuration that achieves a balance between the low cost and the high reliability by using Ethernet® equipment for accommodating user lines and SONET equipment for transmission between user sites.
In this case, a protection scheme such as described below is employed when connecting between Ethernet® equipment and SONET equipment by using a redundant configuration.
In a synchronous network such as a SONET, redundancy is implemented by setting a plurality of paths along a redundantly configured segment, with provisions made to select one of the paths for use.
On the other hand, in a network such as an Ethernet® that determines a path through flooding and address learning, if a plurality of links are provided between two switches as in a synchronous network, frame duplication and looping occur, and consequently, the redundant configuration cannot be implemented by such a method, as described in detail in Japanese Laid-open Patent Publication No. 2005-130408. Accordingly, in the case of Ethernet®, it is usual to implement redundancy by using a link aggregation function that bundles together a plurality of links in a virtual fashion.
The link aggregation function includes a function for distributing frames arriving at a switch to a plurality of links in accordance with a prescribed rule, and a function for collecting frames arriving from a plurality of links onto a single link. If one of the plurality of links fails, failure recovery is performed by changing the distribution rule so that no frame flows to that link (degenerate operation).
FIG. 1 shows a first example of redundantly configuring the connection between the Ethernet® and the SONET; in this example, the connection between the Ethernet® and the SONET is redundantly configured by directly linking the link aggregation of the Ethernet® to the redundant configuration of the SONET. A frame arriving at a port C of the Ethernet® equipment 10 having the link aggregation function is distributed to a port A or a port B according to a prescribed rule, i.e., according to whether the total number of 1 bits contained in the source MAC address and the destination MAC address is odd or even. On the other hand, frames arriving at ports A and B are combined together and sent out from port C.
The port A and the port B are connected to a working port and a protection port, respectively, on the SONET equipment 12. At the SONET equipment 12 side, the signal received via the working port is selected by a switch 13 and sent to the WAN side. Though not shown here, a facility for mapping an Ethernet® frame into a SONET frame to form an EOS (Ethernet Over SONET) frame and for demapping the EOS frame into the Ethernet® frame is provided between the Ethernet® equipment 10 and the SONET equipment 12.
At the SONET equipment 12 side, since the switch 13 is set to select the working port, if a signal is received from the protection port, the signal cannot be sent to the WAN side. Therefore, as shown by reference numeral 14, either the link is forcefully cut off or the light transmission is stopped at the protection port so that all the transmitted signal is received at the working port by the earlier described degenerate operation. If the protection link is not cut off here, part of the signal is transmitted on the protection link because of the link aggregation and, since that part of the signal is not delivered to the WAN side, a signal dropout occurs.
FIG. 2 is a diagram showing a method for recovery when a failure occurs at the working port in the connection shown in FIG. 1.
Upon detecting the occurrence of failure at the working port, the SONET equipment 12 clears the link shutoff state of the protection port and, at the same time, throws the switch 13 to the protection side so that the signal from the protection port can be received. At the Ethernet® equipment 10, the link aggregation function operates upon detection of a failure at the port A, and all the signal is transmitted from the port B. This series of operations provides the protection function.
Since this method uses the link aggregation function generally provided in Ethernet® devices, the advantage is that any suitable device can be used to accommodate users, but since the SONET equipment is unable to receive both the working and protection links simultaneously, and the link on the protection side has to be normally held in the shutoff state, there arises the problem that it is not possible to monitor the protection side to verify its properly functioning state.
FIG. 3 shows a second example, in which the link aggregation function of Ethernet® is also incorporated in the SONET equipment and the connection between the Ethernet® and the SONET is redundantly configured by connecting between them using redundant links implemented by the link aggregation. In FIG. 3, equipment corresponding to the Ethernet® equipment 10 in FIGS. 1 and 2 is not shown.
In FIG. 3, Ether frames received at two Ethernet® ports 16 and 18 are collected by a collector 20 into one frame which is then mapped by a mapping unit 22 onto a SONET frame for output to the WAN side. A SONET frame received from the WAN side is disassembled into Ether frames by a demapping unit 24, and the path is calculated by a path calculator 26; then, each Ether frame is distributed by a distributor 28 to the port 16 or 18.
In this example, the link aggregation function of the Ethernet® is also incorporated in the SONET equipment 12, and the SONET equipment is connected to the Ethernet® equipment by the plurality of links redundantly configured by the link aggregation. Accordingly, the above-described problem of being unable to monitor the protection side to verify its properly functioning state does not occur here, because signals are flowing on all the links as long as the links are functioning properly.
However, there arises the problem that the collector 20, the path calculator 26, and the distributor 28, which together implement the link aggregation function, cannot be configured redundantly though they are Ethernet® devices having low reliability.